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Date:   Wed, 1 Nov 2023 23:28:32 +0100
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Subject: Re: [PATCH v13 17/35] KVM: Add transparent hugepage support for
 dedicated guest memory
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To:     Sean Christopherson <seanjc@google.com>
Cc:     Xiaoyao Li <xiaoyao.li@intel.com>, Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        Huacai Chen <chenhuacai@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Anup Patel <anup@brainfault.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>, kvm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
        linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Xu Yilun <yilun.xu@intel.com>,
        Chao Peng <chao.p.peng@linux.intel.com>,
        Fuad Tabba <tabba@google.com>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Anish Moorthy <amoorthy@google.com>,
        David Matlack <dmatlack@google.com>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        Isaku Yamahata <isaku.yamahata@intel.com>,
        =?UTF-8?B?TWlja2HDq2wgU2FsYcO8?= =?UTF-8?Q?n?= <mic@digikod.net>,
        Vlastimil Babka <vbabka@suse.cz>,
        Vishal Annapurve <vannapurve@google.com>,
        Ackerley Tng <ackerleytng@google.com>,
        Maciej Szmigiero <mail@maciej.szmigiero.name>,
        David Hildenbrand <david@redhat.com>,
        Quentin Perret <qperret@google.com>,
        Michael Roth <michael.roth@amd.com>,
        Wang <wei.w.wang@intel.com>,
        Liam Merwick <liam.merwick@oracle.com>,
        Isaku Yamahata <isaku.yamahata@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
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 <20231027182217.3615211-18-seanjc@google.com>
 <7c0844d8-6f97-4904-a140-abeabeb552c1@intel.com>
 <ZUEML6oJXDCFJ9fg@google.com>
 <92ba7ddd-2bc8-4a8d-bd67-d6614b21914f@intel.com>
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From:   Paolo Bonzini <pbonzini@redhat.com>
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Precedence: bulk

On 11/1/23 17:36, Sean Christopherson wrote:
>>> "Allow" isn't perfect, e.g. I would much prefer a straight KVM_GUEST_MEMFD_USE_HUGEPAGES
>>> or KVM_GUEST_MEMFD_HUGEPAGES flag, but I wanted the name to convey that KVM doesn't
>>> (yet) guarantee hugepages.  I.e. KVM_GUEST_MEMFD_ALLOW_HUGEPAGE is stronger than
>>> a hint, but weaker than a requirement.  And if/when KVM supports a dedicated memory
>>> pool of some kind, then we can add KVM_GUEST_MEMFD_REQUIRE_HUGEPAGE.
>> I think that the current patch is fine, but I will adjust it to always
>> allow the flag, and to make the size check even if !CONFIG_TRANSPARENT_HUGEPAGE.
>> If hugepages are not guaranteed, and (theoretically) you could have no
>> hugepage at all in the result, it's okay to get this result even if THP is not
>> available in the kernel.
> Can you post a fixup patch?  It's not clear to me exactly what behavior you intend
> to end up with.

Sure, just this:

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 7d1a33c2ad42..34fd070e03d9 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -430,10 +430,7 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
  {
  	loff_t size = args->size;
  	u64 flags = args->flags;
-	u64 valid_flags = 0;
-
-	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
-		valid_flags |= KVM_GUEST_MEMFD_ALLOW_HUGEPAGE;
+	u64 valid_flags = KVM_GUEST_MEMFD_ALLOW_HUGEPAGE;
  
  	if (flags & ~valid_flags)
  		return -EINVAL;
@@ -441,11 +438,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
  	if (size < 0 || !PAGE_ALIGNED(size))
  		return -EINVAL;
  
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
  	if ((flags & KVM_GUEST_MEMFD_ALLOW_HUGEPAGE) &&
  	    !IS_ALIGNED(size, HPAGE_PMD_SIZE))
  		return -EINVAL;
-#endif
  
  	return __kvm_gmem_create(kvm, size, flags);
  }

Paolo